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Re: [Cfrg] erratum for hmac what do we think...

"Dang, Quynh (Fed)" <quynh.dang@nist.gov> Thu, 02 February 2017 14:22 UTC

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From: "Dang, Quynh (Fed)" <quynh.dang@nist.gov>
To: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>, Stephen Farrell <stephen.farrell@cs.tcd.ie>, "cfrg@irtf.org" <Cfrg@irtf.org>
Thread-Topic: [Cfrg] erratum for hmac what do we think...
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Date: Thu, 02 Feb 2017 14:22:32 +0000
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Subject: Re: [Cfrg] erratum for hmac what do we think...
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One way to fix that is always hash the key.


Quynh.

________________________________
From: Cfrg <cfrg-bounces@irtf.org> on behalf of Dang, Quynh (Fed) <quynh.dang@nist.gov>
Sent: Thursday, February 2, 2017 9:20:03 AM
To: Paterson, Kenny; Stephen Farrell; cfrg@irtf.org
Subject: Re: [Cfrg] erratum for hmac what do we think...


Kenny just made a great point that HMAC is not a PRF: it is trivial to find 2 keys which produce the same HMAC output.


Quynh.


________________________________
From: Cfrg <cfrg-bounces@irtf.org> on behalf of Paterson, Kenny <Kenny.Paterson@rhul.ac.uk>
Sent: Thursday, February 2, 2017 9:07 AM
To: Stephen Farrell; cfrg@irtf.org
Subject: Re: [Cfrg] erratum for hmac what do we think...

Dear CFRG,

It'd be great if some HMAC experts could take a look at this proposed
erratum and give a view on it.

I looked quickly myself. It's an undesirable property, but I don't think
it's disastrous (yes, I could invent scenarios where one could come
unstuck because of it). It reminds me somewhat of the well-known, and
again somewhat unfortunate, fact that HMAC keys of different lengths can
end up being padded to form colliding keys.

Cheers,

Kenny

On 02/02/2017 02:24, "Cfrg on behalf of Stephen Farrell"
<cfrg-bounces@irtf.org on behalf of stephen.farrell@cs.tcd.ie> wrote:

>
>Hiya,
>
>There's an erratum posted for hmac [1] where I'd be
>interested in what folks here think.
>
>I'm unsure if this is a real problem, esp given that
>there are I guess a lot of implementations.
>
>And even if it were a real problem, I'm not sure we'd
>want that fix.
>
>Opinions welcome...
>
>Thanks,
>S.
>
>[1]
>https://www.rfc-editor.org/errata_search.php?rfc=2104&eid=4809&rec_status=
>15&area_acronym=&errata_type=&wg_acronym=&submitter_name=&stream_name=&sub
>mit_date=&presentation=records
>

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